A radio frequency (RF) integrated circuit may include multiple transistor dies. A die attach machine is used to place the transistor dies in an integrated circuit package. The die attach machine attempts to place each die in its appropriate position within the package. However, every die attach machine has a specified tolerance allowing some variance in the placement of the dies. Thus, the actual position of a given die within the package may differ from its ideal position by an amount less than or substantially equal to the tolerance. A robotic bonding tool may then be used to wire-bond the dies to other circuit elements within the package, and to leads of a package leadframe. Such a tool generally includes a surface/wire-feed detection system that detects bond pads or other bonding sites of a given die, and determines the height coordinates of these bond pads. The other circuit elements in an RF integrated circuit may include, for example, tuning capacitors.
A wire bond profile may be characterized as a side or profile view of a wire extending from a first bond site to a second bond site. In an RF integrated circuit, the wire bonds may carry high frequency signals. Certain types of RF integrated circuits, such as RF power transistors, are tuned through these wire bond profiles.
Since the placement of a transistor die within the package can vary from one attach series to the next due to the die attach machine tolerance, an associated variation in the RF performance of the circuit may result. The variation in RF performance may be caused by unequal areas under wire bond profiles connecting similar elements of the integrated circuit. In conventional practice, the wire bond length is generally held constant regardless of the bond distance between a given pair of first and second bond sites. Therefore, as the bond distance changes, the area under the wire bond profile changes, causing inconsistent RF performance from package to package.
Attempts to compensate for the tolerance of the die attach machine include increasing the size of the bond pads so that while wire length remains constant and the distance varies between bond sites, the wire may still be bonded on the bond pad without greatly affecting the loop height and the resulting area under the wire bond profile. Bond operations may be performed on a portion of the bond pad closer to the perimeter of the die or on a portion of the bond pad closer to the interior of the die. However, many die attach machine tolerances are too great to be fully compensated for by the size of the bond pad. Further, larger bond pads take up valuable room on the surface of the die that may be used for circuitry. Finally, die attach machines with very low tolerances greatly increase the cost of packaging.
Traditional bonding tools have also been equipped with mechanical features called “close at loop” and “close at bond.” Close at bond is the standard clamp close position in which the clamps will close after the second bond contact. Close at loop will close the wire clamps at the peak position of the looping trajectory, reducing the variation from wire to wire. However, such features are purely mechanical and associated with a wire clamp mechanism. Since no actual calculation of the area under the wire profile is made using the bond distance, the process is not very accurate or repeatable.
Thus, a need exits for improved wire-bonding techniques, particularly in RF integrated circuit applications.